1. Field of the Invention
This invention relates to semisubmerged, small water plane area twin hull (swath) ships, specifically to steerable bow thrusters which draw seawater in from the sides of the pontoons, and discharge it from a steerable nozzle on the top of the pontoons. The nozzles are directional, and allow the swath to remain in one place without anchoring, also known as station-keeping.
2. Description of Prior Art
Small water plane area twin hull (swath) vessels, also called semisubmerged vessels, have been developed for improved motion, seakeeping and performance characteristics in high sea states. Many U.S. patents including U.S. Pat. No. 234,794 to Lundborg (1880), U.S. Pat. No. 3,063,397 to Boericke (1962), U.S. Pat. No. 3,623,444 to Lang (1971) and U.S. Pat. No. 4,174,671 to Seidl (1979) have been granted which disclose ships of this configuration.
Each of the configurations noted includes submerged pontoons that are parallel to each other and the design water line, vertical struts of reduced waterplane area, and an upper bridging deck. The above patents show a number of configurations for swath vessels.
Thruster systems have been developed for maneuvering and/or propelling a marine vessel by means of pump, tunnel prop, pipe and valve arrangements which take water in from the sea and discharge the water through outlets located in the boat hull so as to achieve the desired propulsive or turning effect. Such systems, as described for example, in U.S. Pat. Nos. 3,517,633, 4,056,073, and 4,214,544 typically employ pipes of various cross sections which curve or join other pipes or nozzles to form the water transport path. Thruster systems have also been developed which employ propellers of many types including screw, cycloidal or shrouded impellers on drive units that extend out of the vessel's hull as shown in U.S. Pat. Nos. 4,294,186 and 4,732,104.
All of these systems designed for conventional monohull vessels employ the bottom or side of the hull for locating the propulsive device. The swath hull form provides a new alternative that is superior for a number of reasons. The new alternative is to locate the propulsive device on the top side of the submerged pontoons. By locating the propulsive device on the top side of the submerged pontoons rather than on the bottom side, the draft of the vessel is minimized and damage to the propulsive device is eliminated in the event of grounding. In shallow water, the water on the top side of the pontoons is also likely to have less foreign matter that can foul or damage the propulsive device than the seawater on the bottom side of the pontoons due to its distance away from the sea bottom.
Conventional monohulls today employ rotatable propellers which extend downward from the bottom of the hull. Pursuant to the advancements disclosed in this patent an improvement of that system for swath vessels would be to invert the outdrive unit and attach it to the upper side of the submerged pontoon. Conventional monohulls today also employ tunnels with pumps or shrouded propellers which pull water in from the sides and discharge it out the bottom through a grating with louvered vents. The grating can be rotated to direct the thrust in approximately the desired direction. The direction is approximate because of the sharp change of direction the water must make through the relatively inefficient vents. Much of the thrust is spilled downward instead of aft, forward or athwartship. In accordance with the ideas advanced in this patent this system would also be improved for swath vessels by inverting the discharge and directing it out the top of the pontoon.
For a swath vessel operating in shallow draft areas, additional advantage is gained with respect to environmental impact. The environmental community has recognized the biological and ecological importance of the ocean bottom and the negative impact of sediment transport due to propeller wash. This is particularly relevant for vessels operating in and around shallow tropical reefs. Locating the propulsive device on top of the pontoons for swath vessels moves the propeller wash well away from the ocean bottom and even provides the pontoon itself as a barrier between the propeller wash and the ocean bottom greatly reducing and even eliminating the adverse environmental impact.
This advantage is even more apparent when one considers a vessel that is carrying snorkel and scuba divers to a shallow tropical reef. Such a vessel is discouraged from employing anchors due to the damage they will inflict on the fragile reef. Presently, permanent moorings are sometimes installed on reefs to allow vessels to visit, but they involve a substantial effort and expense to install and maintain. They are also of limited value to larger vessels due to the limited strength of the coral formations and coral substrate to which the moorings are attached.
The ideal solution for these vessels would be to not attach to the reef in any way whatsoever but rather station-keep above the reef. A vessel is station-keeping when it is maintaining a single location without mechanical attachment to the seafloor usually through the use of thrusters. Presently, many vessels without thrusters will drift with the wind and current for a period of time and then occasionally motor back to the area of the dive group. Ideally, the vessel would be under control at all times and either station-keep over one particular spot on the reef or slowly follow the dive group as it swims across the reef. For larger vessels, use of the main engines for this purpose is usually not satisfactory as the propeller wash would disturb the sediment on the ocean bottom reducing visibility. Any vessel using exposed propellers also endangers swimmers or divers in the water with the rotating propeller blades. Additionally, the use of the main propulsion engines to station-keep or maneuver the vessel with top speeds up to two knots would not be a very efficient use of engines that are designed to move the vessel at much greater speeds.
A conventional monohull with a rotatable bow thruster fitted to the bottom would station-keep but would still have the problem of propeller wash disturbing the sediment and reducing underwater visibility. Depending on the specific propulsive device utilized, rotating propellers may also pose a hazard to swimmers and divers. Pursuant to the ideas disclosed in this patent, a swath vessel with the propulsive device mounted on top of the pontoon would not at any time disturb the bottom sediments. By shrouding the propulsive device, danger for swimmers and divers would also be eliminated.
Mounting of the propulsion device on top of the pontoon rather than on the side has additional mechanical benefits. Top mounting of the propulsion device allows thrust in any horizontal direction by simply rotating the propulsive unit about a vertical axis. If the propulsive unit were installed on the side of the pontoon, the thrust would be limited to fore and aft direction with no athwartship capability unless additional range-of-motion capability were added to the propulsion device. Additional range-of-motion capability would inherently be more complex leading to additional hardware and sealing problems. Also, installation of a propulsion device on a side of a pontoon limits the thrust to only one side of the pontoon. On a two pontoon swath, a side installation would make only one of two thrusters available for athwartship movement.
The advancement described in this patent has additional features. Shrouding the propulsive device and ducting the thrust through piping provides the greatest level of safety for swimmers and divers. Seawater intake or suction drawn from the side of the pontoons will allow a swath vessel to maneuver even if it is temporarily ballasted up so that the upper side of the pontoons are clear of the water. Sea water intakes on the side and a rotating nozzle on top of the pontoon for the sea water discharge as opposed to a rotatable propeller will eliminate cavitation or emmersion of the propeller in a seaway.